Electromagnetic sensor system

ABSTRACT

A vehicle ( 160 ) such as a motor road vehicle is fitted with an electromagnetic sensor system comprising transmitting means (e.g.  4, 1 ) for transmitting a radio frequency signal, receiving means (e.g.  2, 6 ) for receiving reflections of said signal from remote objects, sampling means ( 20, 28 ) operable to sample the received reflected signal (or a signal derived therefrom), and processing means (e.g.  36 ) for processing the sampled signal, and operable to detect said reflections in the sampled signal, and to determine information on the presence, position and/or range of said object. The system includes filter means (e.g.  34 ) for preventing radio signals transmitted by other sources or noise spikes from causing interference which results in spurious detections or indications of range by the processing means.

FIELD OF THE INVENTION

This invention relates to an electromagnetic sensor system, and isparticularly concerned with dealing with various types of noise whichcan be received by such a system. The system may be adapted for shortrange obstacle sensing. Such a system may be installed on a motor roadvehicle as part of a collision warning system.

BACKGROUND OF THE INVENTION

A known type of electromagnetic sensor system uses a series of broadband radio frequency pulses to detect the presence and/or motion ofobjects. Such a system has a pulse generator forming part of atransmitter for transmitting a train of radio frequency pulses. Echoesof those pulses are received by a receiving antenna, the output of whichis sampled by a sampler at a succession of sampling periods, eachoccurring at a predetermined delay after the transmission of arespective pulse. If the reflection of a pulse is received during agiven sampling period, this is indicative of the pulse having travelledto the object and returned to the receiver in the predetermined delay,so that it can be deduced that the object is entering or leaving anotional range gate or envelope surrounding the transmitter andreceiver.

Examples of such systems are shown in U.S. Pat. No. 5,361,070 (McEwan)and European Patent No EP 469027B (Cambridge Consultants Limited).

In general, the magnitude of the reflected pulses can be small inrelation to background noise, and as a result the signals received overthe sampling periods can be averaged in order to improve the signal tonoise ratio, as is discussed in U.S. Pat. No. 5,361,070.

However such a system is still susceptible to interference from RF spikenoise (produced by other systems of the same type, for example) andcontinuous wave RF signals.

These latter problems are particularly relevant where the system is tobe installed in a motor road vehicle, other vehicles may be equippedwith similar systems, which generate the noise spikes, and the system islikely to be operated in the vicinity of various sources of continuouswave signals, such as mobile telephone apparatus/vehicle identificationsystems.

The signals strengths of an echo received by an electromagnetic sensorsystem from a target of cross-section e at range R is given by thefollowing expression:

S=PGAσ/(4πR ²)²

where PGA is power-gain-area product for the system.

If the target is equipped with a similar system, then there is thepossibility that a pulse transmitted by the target will arrive at thereceiver during a sampling period. The signal strength of such aninterfering pulse is:

I=PGA/(4πR ²)

so that the signal-to-noise ratio, SNR, is:

SNR=σ/(4πR ²)

For an automotive application typical values might be σ=0.1 m²·R=30 mgiving:

SNR=−50 dB

compared with a required signal-to-noise ratio typically of at least +15dB for acceptable detection performance.

This signal-to-noise ratio will be improved by averaging the signalsreceived over a large number of sampling periods, but even if 10⁴samples are averaged, the processing gain will only be 40 dB. Even ifonly one interfering pulse is received per averaging period, theaveraged SNR will be −10 dB which might still be too low.

An impulse modulated electromagnetic sensor system operates at a pulserepetition frequency (PRF) typically of order 1-10 MHz: both the pulsegenerator in the transmitter and the sampler in the receiver willoperate at this frequency. The audio frequency (AF) output from thesampler will have a bandwidth from DC to the Nyquist frequency (one halfof the PRF).

For many applications, and in particular for automotive applications,the target echo occupies only a fraction of the AF bandwidth. The echobandwidth is related to the wavelength of the signature (typically 3 to30 cm) and the relative speed of the target (say 0 to 100 m/s) fromwhich the pulses are reflected, giving a bandwidth of order 3.3 kHz. Theoutput signal processing can include a low-pass filtering stage toimprove the signal-to-noise ratio by rejecting noise (eg thermal noisein the receiver) outside the AF band.

The receive antenna will pick up any external RF signal, for examplefrom a nearby radio transmitter. Of particular concern in automotiveapplications are on-car or roadside transmitters such as mobile phonesor tolling vehicle identification systems. RF filters in the receivercan suppress any signal outside the operational bandwidth of the system,but other signals will be aliased into the AF output. For example, ifPRF is 1 MHz and the operational bandwidth includes frequencies around2GHz, then a signal at 2.000001 GHz will be aliased to 1 kHz and asignal at 2.000100 GHz will be aliased to 100 kHz. The low-pass filter(presumed DC to 3.3 kHz) in the output processing will reject the secondsignal but not the first.

In practice, modulated RF signals have finite bandwidth, typically 100kHz for GSM systems, with corresponding channel separations of 200 kHz.An FM signal with a centre frequency of 2 GHz will appear in the AFoutput as energy distributed over the DC−50 kHz band, and a proportionof this energy will be passed by the low-pass filter. A signal with acentre frequency around 2.0002 GHz will appear in the AF output asenergy distributed over 150-250 kHz, and should be rejected by the lowpass filter.

Thus, in an impulse modulated electromagnetic sensor system thereflected signature from a target can be contaminated by continuous wave(CW) or amplitude/frequency /phase-modulated continuous wave(narrowband) RF signals which appear in the AF output. If theiramplitude is sufficiently large they will obscure the wanted echo andprevent the target from being detected.

The invention seeks to provide various methods and apparatuses which areless susceptible to interference by either or both these types of noise.

BRIEF DESCRIPTION OF THE INVENTION

According to a first aspect of the invention, there is provided anelectromagnetic sensor system comprising transmitting means fortransmitting a train of radio frequency pulses, receiving means forreceiving reflections of said pulses from remote objects, sampling meansfor sampling the output of the receiving means, processing meansconnected to the sampling means, and operable to detect said reflectionsin the sampled signal, and to determine information on the presence orrange of said object, and gating means for preventing radio pulsestransmitted by other sources or noise spikes from causing interferencewhich results in spurious detections or indications of range by theprocessing means.

Thus, a number of such systems may be used in proximity to each othersince the gating means prevents the pulses transmitted by one of thesystems being mistaken by another system for reflections of pulsestransmitted by that other system. Such mistakes would otherwise giverise to spurious detections or other inaccuracies in the output of theprocessing means.

The gating means may be arranged to operate by monitoring signalsreceived by the receiving means, and preventing or inhibiting theoperation of the sampling means when the amplitude of said signalsexceeds a threshold.

As is explained above, reflections of signals transmitted by thetransmitting means will make a smaller contribution to the amplitude ofthe output of the receiving means than will transmitted pulses receiveddirectly, (i.e. without an intervening reflection) from other systems.Consequently, the above thresholding procedure will discriminate betweenmost genuine reflections and other pulses, which are transmitted byother systems and which are received directly by the receiving means.

Preferably, the gating means comprises a threshold detector fordetermining whether the received signal is above said threshold and, ifit is not, generating an enabling signal for enabling the sampler tooperate, and delay means for delaying the passage of the received signalfrom the receiving means to the sampling means so that the operation ofthe threshold detector and the arrival of the signal at the samplingmeans are synchronised.

Alternatively, the filter means may be operable selectively to limit theamplitude of the input to the sampling means by truncating any peaks inthe sampled signal which would otherwise exceed said threshold, so thatthe maximum amplitude of the input to the sampling means corresponds tosaid threshold.

In this case, the filter means allows the transmitted pulses receiveddirectly from other systems to reach the processing means, but ensuresthat those signals are inhibited so as to cause less seriousinterference.

The level of the threshold is ideally such that all transmitted pulsesreceived directly from other systems, but as few as possible of thegenuine reflected pulses exceed the threshold.

In either case the filtering means preferably further comprisesthreshold setting means for adjusting the threshold applied by thethreshold detector to ensure that the majority of genuine reflectedpulses lie below the threshold.

Preferably, the threshold setting means is operable to analyse thestatistical distribution of the amplitudes of the noise received by thereceiving means at a plurality of sample times, and to set the thresholdat a level which is dependent upon the spread of the distribution.

To that end, the threshold setting means is preferably connected to theoutput of the sampling means.

In that connection, the threshold setting means may be so arranged as toset a threshold which is approximately three standard deviations abovethe mean of said distribution.

The system may be so arranged that if the statistical behaviour of thenoise exhibits a given characteristic a warning signal is generated.

Thus, the threshold setting means can take account of variations in thebackground noise levels which might otherwise result in the applicationof a threshold which is too high or too low.

The system may be adapted for installation on a motor road vehicle, andmay be operable to detect the approach of objects or other vehicles tosaid vehicle, the system further including alarm means connected to thesignal processing means for warning the driver of any risk of collisionwith said obstacles or other vehicles.

According to a second aspect of the invention, there is provided anelectromagnetic sensor system comprising transmitting means fortransmitting a train of radio frequency pulses, receiving means forreceiving reflections of said pulses from remote objects, sampling meansfor sampling the output of the receiving means during each of asuccession of sampling periods, processing means for analysing theoutput from the sampling means to provide an indication of at least thepresence of any such object and its distance from the receiving means,the system also including filtering means for detecting intermittentcontinuous wave radio frequency signals of frequencies which would causeinterference in the sampler output and, in response to any suchdetection, preventing or inhibiting the operation of the system so thatsaid continuous wave signals do not give rise to spurious detections ofobjects or incorrect indications of distances by the processing means.

This aspect of the invention exploits the fact that many radiocommunications systems use non-continuous transmission. For example, amobile phone operating in accordance with a Time Domain Multiple Access(TDMA) protocol operates with a 1-in-7 duty cycle at 270 Hz. Thus, ifthe mobile telephones' allocated radio channel falls into theoperational band of the impulse electromagnetic sensor system, thefilter means will. in effect, blind the system for approximately 4 ms inevery 30 ms. This corresponds to relatively small loss of informationwhich can be readily accommodated by the processing means.

The filtering means may be arranged Lo inhibit the operation of thesystem whenever RF energy of a frequency lying in a predetermined bandis detected. Alternatively, the filtering means may be arranged todetermine the periodic energy pattern in such an RF signal, and toinhibit the system when interference is due to be present.

Preferably, the filtering means is operable to inhibit the operation ofthe system by interrupting the operation of the sampling means, orlimiting the amplitude of the output sampling means.

Preferably, the filtering means is connected to the receiving means andis operable to detect said continuous wave RF signals by monitoring theoutput to the receiving means.

The system may include delay means connected in series between thesampling means and the receiving means, and downstream of the connectionof the filtering means, in order to synchronise the operation of thefiltering means with the supply of the signal to the sampling means.

However, the aforementioned feature can be dispensed with where thefiltering means determines the frequency of occurrence of the continuouswave signal and in effect anticipate when the signal will be present.

Conveniently, the filtering means comprises integration means fordetermining the total energy of RF signals, within said band offrequencies, received by the receiving means over a given integrationperiod, the filtering means being operable in response to an increase insaid detected energy, said increase being indicative of the presence ofa continuous wave interference signal.

According to a third aspect of the invention there is provided a methodof operating an electromagnetic sensor system, the method comprising thesteps of transmitting a train of radio frequency pulses at a given pulserepetition frequency, causing a sampler to sample the output of areceiver at each of a succession of sampling periods occurring at asampling frequency which is a multiple of the pulse repetitionfrequency, forming from the output of the sampler a first channel in thespectrum of which any continuous wave interference radio signal of afrequency which is a multiple of the sampling frequency and/or the pulserepetition frequency is aliased to a band in the sampler output in whichthe reflected signals appear, forming a second channel in the spectrumof which said continuous wave interference signal is either aliased intoa higher band or into the same band as in the first channel, dependingon the frequency of the interference signal, and in the former caseoutputting a signal by filtering the second channel to remove any suchinterference signal and in the latter case outputting a signal bycombining the first and second channels in such a way as to cancel outany such interference signal appearing in said band of the reflections.

In practice, the reflected signals would appear in a number of bands inthe sampler output, including a band from DC to a relatively lowfrequency (for example 3.3 kHz). A continuous wave interference signalof a radio frequency which is an odd multiple of the sampling rate willbe aliased to a much higher band of frequencies, and can therefore beremoved by low pass filtering of the second channel. However, acontinuous wave interference signal of a frequency which is an evenmultiple of the sampling rate will be aliased to the same low frequencyband. However, such since a signal also appears in the first channel, itcan be cancelled out by, for example, subtraction of one channel fromthe other (if the amplitude of the aliased signal is substantially thesame in each channel).

Preferably, the step of forming the first channel comprises selectingsamples only for periods in which any reflections of transmitted pulsesin the range of the system cannot be received, the second channel beingformed from all the samples produced by the sampler.

Where the channels are to be processed digitally, preferably the formingof the first channel further comprises a step of creating additionalsamples by a process of interpolation from the selected samples so that,the first and second channels have the same sampling rate.

Conveniently, the sampling rate is twice the pulse repetition frequency,the first channel being formed from samples obtained during alternatesampling periods.

Preferably, the method comprises the further steps of generating a thirdchannel by subtracting the first channel from the second channel,comparing the amounts of energy in the second and third channels, andforming an output signal from which whichever of those channels has thelower energy.

According to this aspect of the invention, there is also provided anelectromagnetic sensor system for performing the aforementioned method,the system comprising transmitter means for generating a series of radiofrequency pulses at a given pulse repetition frequency, receiving meansfor receiving reflections of said pulses from objects in the remainderof the system, sampling means for sampling the output of the receivingmeans at a rate which is a multiple of the pulse repetition frequency sothat some of the samples occur periodically when reflections of thetransmitted pulses in the range of the system are not received by thereceiving means, signal processing means for forming a first channelfrom said some samples, a second channel from all the samples in theoutput of the sampler, and a third channel formed by subtracting thefirst channel from the second channel so as to remove any continuouswave interference signal which has been aliased in the output of thesampling means to the band in which any reflections are present, theapparatus further comprising comparator means for determining which ofthe second and third channel contains the least energy and forming anoutput from that channel.

According to a fourth aspect of the invention, there is provided anelectromagnetic sensor system comprising transmission means fortransmitting a train of radio pulses, receiving means for receivingreflections of said pulses, sampling means for sampling the output ofthe receiving means over a succession of sampling periods, eachcorresponding to a respective pulse, detection means for detecting thepresence of any continuous wave radio interference signal and controlmeans for so controlling the pulse repetition frequency and samplingrates at any such interference signal is aliased, in the output of thesampling means, to a band of frequencies outside the band in which saidreflections appear.

Preferably, the control means is operable to control the sampling meansand the transmission means in such a way that the sampling rate is thesame as the pulse repetition frequency, the control means also beingoperable to select said frequency so that the interference signal isaliased to a higher band of frequencies than that in which thereflections appear.

In this case, the apparatus preferably includes a low pass filter forremoving said interference signal from the sampler output.

According to this aspect of the invention, there is also provided amethod of operating an electromagnetic sensor system, the methodcomprising the steps of transmitting succession of radio pulses,sampling the output of a receiver, arranged to receive reflections ofsaid pulses, over a succession of sampling periods at a given samplingrate, determining whether the receiver has received any continuous waveinterference signal which would be aliased into the band of the sampleroutput in which said reflections appear and, if such a signal has beendetected, altering the pulse repetition frequency and sampling rate sothat the interference signal is aliased to a band outside that in whichthe reflected signals appear.

Preferably, the pulse repetition frequency and sampling rate are chosenso that any such interference signal is aliased substantially to theNyquist frequency in sampler output.

Preferably, however, the interferer is aliased to a frequency just belowtile Nyquist frequency so that at least a first harmonic of theinterference signal (and preferably also the second and third harmonics)also lie outside the band in which the reflected signals appear.

The pulse repetition frequency and sampling rate can be controlled toany value in a continuum. Alternatively, the pulse repetition frequencyand sampling rate can be a selected one of a number of predeterminedfrequencies for any given band of interference signal.

According to a fifth aspect of the invention, there is provided anelectromagnetic sensor system comprising transmitting means fortransmitting a train of radio pulses. receiving means for receivingreflections of those pulses, sampling means for sampling the output ofthe receiving means over a succession of sampling periods, andprocessing means for processing the output of the sampling means todetermine information on the presence or range of objects from which thepulses are reflected, wherein the sampling means comprises a pluralityof samplers and filtering means for band-pass filtering the signalreceived by the receiving means so that each sampler is supplied with anRF signal lying in a respective one of a number of frequency bands, thesampler being operable to select for analysis the sampler output whichhas the lowest energy.

The invention also lies in a vehicle fitted with a system in accordancewith any of the preceding aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 is a block diagram of a basic impulse modulated electromagneticsensor system;

FIG. 2 is a block diagram of a first embodiment of impulse modulatedelectromagnetic sensor system in accordance with the first aspect of theinvention;

FIG. 3 is a block diagram of a second embodiment of such a system:

FIG. 4 is a block diagram of an embodiment of an impulse modulatedelectromagnetic sensor system in accordance with the second aspect ofthe invention;

FIG. 5 is a block diagram of a first embodiment of an impulse modulatedelectromagnetic sensor system in accordance with the third aspect of theinvention;

FIG. 6 is a block diagram of a second embodiment of system also inaccordance with the third aspect of the invention;

FIGS. 7 to 16 are graphs illustrating the processing of various signalsreceived by either of the systems shown in FIGS. 5 and 6;

FIG. 17 is a block circuit diagram of an impulse modulatedelectromagnetic cut sensor system in accordance with the fourth aspectof the invention;

FIG. 18 is a block circuit diagram of an impulse modulatedelectromagnetic sensor system in accordance with the fifth aspect of theinvention; and

FIG. 19 shows a car fitted with any of the embodiments of system.

DETAILED DESCRIPTION

FIG. 1 shows a basic type of impulse modulated electromagnetic sensorsystem which does not have the noise countering facilities of thesystems according to any aspect of the invention. This system has atransmitter 200, which is the same as the transmitter of the othersystems described below and which carries an antenna 201 to transmit atrain of radio frequency pulses in response to control signals from asystem clock 202 also of the type used in the other systems.

The clock 202 also triggers a sampler 204 which forms part of a receiver206. The sampler 204 is connected to a second antenna 208 through a setor RF filters 210 and therefore periodically samples the signal receivedby the antenna 208 and filtered by the filters 209. The sampler outputwill thus include reflections of the transmitted pulses if the samplingoccurs when those reflections are present in the received signal. Insuch a case, the timing of the sampling is related to the range at whichthe reflections occur, as discussed below in relation to the systemshown in FIG. 2.

Such reflections arc detected by a signal processor 212, which includesa low pass filter 218 and detection circuitry 219 for informing a maincontrol processor 214 of said detection. The control processor 214, inturn, calculates the range of the reflections, and 214 also sendscontrol signals to the clock 202 so as to control the timing of theoperation of the sampler 204 in relation to the operation of thetransmitter 200 and sends signals to a Man Machine Interface 216. ManMachine Interface comprises an alarm which is triggered if a reflectionis detected at or below a predetermined minimum range.

The receiver 206 can also receive other signals eg noise, any pulsesgenerated by other electromagnetic sensor systems and any continuouswave signals generated by, for example, mobile telephone apparatus.Whilst some of these signals will be removed by RF filters 210 and thelow pass filter 218, others will not and can in some circumstances causespurious detections of reflections or incorrect calculations of range.

Such signals are dealt with in various different ways by the systemsdescribed below.

1. Thresholding

FIG. 2 shows an impulse electromagnetic sensor system which is adaptedto be installed in a motor road vehicle, and which includes a transmitantenna 1 and a receive antenna 2 both of which are adapted to bemounted at a suitable position on the vehicle, for example, in the rearbumper (as shown in FIG. 19).

The antenna 1 is connected to a transmitter 4 which is operable to causethe antenna 1 to transmit a train of radio frequency pulses. Each pulsesubstantially consists of only a few cycles of electro magnetic energy.In the present case, the pulses are each of approximately 0.1-10nanoseconds' duration, and the pulses are generated at intervals in theregion of 0.1-10 microseconds such that the frequency of occurrence ofthe pulses (i.e. the pulse repetition frequency) is in the range of onehundred thousand to ten million pulses per second. The rf frequencies ofthe train of pulses lie in the range of 100 MHz-10 GHz, and each pulsehas a rise time (i.e. the time taken for the pulse amplitudes to reachits peak from a quiescent level) of the order of tens of picoseconds to10 nanoseconds.

Those pulses are reflected by objects or other vehicles near the vehiclein which the system is installed, and the reflections are received bythe antenna 2 which is connected to a receiver 6 which periodicallysamples the signals being received by the antenna 2. The received signalis sampled at each of a succession of sampling periods, each of asimilar duration to that of the transmitted pulse. Each of the sampledperiods occurs at a predetermined delay (normally of 1 to 200nanoseconds) after the transmission of a corresponding pulse by theantenna 1. If the antenna 2 receives the reflection of the correspondingtransmitted pulse during a given sampling period, this is indicative ofthe transmitted pulse having travelled from the antenna 1 to areflecting surface and back to the antenna 2 in the predetermined delay.Thus, the receipt of such a reflection is indicative of a surface lyingon a range gate or shell which is defined by the condition that thecombined distances from the antenna 1 and the antenna 2 to a given pointon the gate or shell is the same as the distance which will be travelledby the transmitted and reflected pulse in the predetermined delay. Thus,if the antennas 1 and 2 are mounted at opposite end regions of thevehicle bumper, the range gate or shell will be in the shape of anellipse, having the two antennas at its foci (for example as shown at162 in FIG. 19).

The operation of the transmitter 4 and receiver 6 is controlled by meansof a control unit 8. The unit 8 is connected to a system clock and pulsegenerator 10 which is operable to send a transmit control pulse alongthe line 12 to the transmitter, and a sampling control pulse along theline 14 to the receiver 6. The line 12 is connected to a pulse generator16 which forms part of the transmitter 4.

The RF pulse generated by the pulse generator is fed to the antenna 1via RF filters 20 which remove from the transmitted pulses frequencieswhich would otherwise interfere with nearby radio or telecommunicationsequipment in the same or other vehicles.

The sampling control pulses supplied along the line 14 are eachgenerated after a predetermined delay from the transmission of acorresponding transmit control pulse, and each sampling control pulse isfed and to an input of an AND gate 26, the other input of which isconnected to the output of the threshold detector 22. The output of thegate 26 is connected to a sampler 28. The threshold detector 22 andsampler 28 also have inputs which are connected to the receive antenna 2via a set of RF filters 30 which remove interference from continuouswave sources, for example, radio broadcasts or mobile telephonetransmissions, from the signals received by the antenna 2. The thresholddetector 22 is connected directly to the filters 30 whilst the sampler28 is connected to the filters 30 via a delay line 32 which may comprisea piece of transmission line of a suitable length.

If the signal received by the antenna 2 exceeds a threshold applied bythe detector 22, the latter generates an inhibit pulse which is fed tothe input of the gate 26. If the threshold is not exceeded, no suchpulse is generated.

The delay associated with the delay line 32 is equal to the delaycreated by the threshold detector 22 and the gate 26, so that theportion of the delayed RF signal received by the sampler 28 from thedelay line 32 when the sampler 28 receives a triggering signal from thegate 26 is the same as the portion of the signal on which the thresholddetector 22 operated.

If any inhibit pulse is generated the gate 26 does not relay anytriggering signal to the sampler 28 so that the latter does not samplethe signal from the receive antenna if that signal exceeds thethreshold. The sampler 28 averages the signals obtained by the sampling(for any given predetermined delay) so that the output from the sampleris an average of, for example, 10⁴ samples. This averaging helps todiscriminate the reflections from random background noise, which willtend to be suppressed by the averaging process.

The output from the sampler 28 is connected to signal processorcircuitry 36, similar to the processor 212 of FIG. 1, which analysesthat output, to determine whether any reflections of pulses have beenreceived.

The circuitry 36 can also perform a frequency analysis of the sampledsignal which can be used to determine the relative speed of a reflectivesurface normal to the associated range gate or shell. The processingcircuitry 34 supplies information back to the control unit 8 which candetermine, from the object's position relative to the antennas on thevehicle or from its relative velocity whether there is a risk of acollision, and if so, will trigger an alarm (not shown), to warn thedriver.

If the antenna 2 receives transmitted pulses directly from other impulsemodulated electromagnetic sensor systems, these will generally be of ahigher amplitude than the reflections of the signals from the antenna 1.Thus, if an appropriate threshold is set, most of the pulses receivedfrom other systems will be above the threshold whilst the majority ofgenuine reflected pulses will be below the threshold.

The threshold is set by means of adaptive threshold setting circuitry 34which adjusts the threshold level so that large amplitude interferencepulses are not sampled, but genuine reflections, background noise, etcare included. To that end, the circuitry 34 analyses the statisticaldistribution of amplitudes of signals sampled by the sampler 28 whichwill generally be a gaussian distribution. The circuitry 34 then sets athreshold equivalent to approximately three standard deviations abovethe mean of the amplitude. All the higher amplitude interfering pulseswill consequently be excluded from sampling and, due to statisticalvariations in noise levels, approximately 0.3% of the genuinereflections will also exceed the threshold and therefore be excluded.However, it can be shown that the omission of the small proportion ofgenuine reflections has a negligible effect on the performance of thesystem as a whole. In addition, low amplitude interference pulses, whichcannot be excluded by this thresholding technique, also have anegligible effect on the system performance.

A further discussion of the threshold setting process follows.

The “adaptive threshold setting” process determines the statisticaldistribution of the sampler (AF) output, under conditions so that it isknown either that no target signal is present, or that the target signalcan be ignored. For example, there could be a dedicated receiver channelwhich is sampled prior to the transmitted pulse. or the transmittedpulse could be periodically inhibited. The raw sampler output could alsobe used, provided that the mean value of that data (which corresponds tothe target echo) varies only slowly and provided that the variabilityabout the mean is independent of the mean level.

The statistics of the sampler output can then be used to determine therequired threshold, correcting for any effect of the sampler itself. Forexample, if the standard deviation of the sampler output is 1 μV, andthe sampler effectively averages the RF data over 16 samples. then wecan infer that the standard deviation of the RF data is 16×1 μV=4 μV.Setting the threshold to 12 μV (3 standard deviations) will enablespikes to be eliminated. Some data will also be lost due to thethresholding process, but less than 0.3% with the threshold set at thislevel.

At short ranges or for strong targets the basic thresholding algorithmdescribed above may cause the echo signal to be truncated. There maytherefore need to be a minimum threshold.

It will be appreciated that, by appropriate generation of samplingcontrol pulses, the system can be used to set a number of range gates.In such cases, a single threshold may be set for all range gates toreduce the component count. Alternatively. however, the threshold can beset independently for different range gates, for example, if thereceiver circuitry is also connected to other antennae with differentfields of view and hence different noise characteristics from theantenna 2.

The system shown in FIG. 3 is similar in many respects to that shown inFIG. 2. Accordingly, the features which correspond to those of thesystem shown in FIG. 2 are denoted by the reference numbers of FIG. 2raised by 100.

Thus, the second embodiment of system has a transmitter 104 which causesa transmit antenna 101 to generate a series of pulses. reflections ofwhich are received by a receive antenna 102 connected to a receiver 104.However, instead of the threshold detector 22, delay line 32 and ANDgate 26, the receiver has a modified sampler 150 which samples thesignal received by the antenna 102, after filtering by RF filters 130,every time the sampler 150 receives a sampling control pulse along theline 114 from the circuitry 110.

When an interference pulse is received directly from another system,therefore, the sampler 150 is not disabled. However, the sampler 150does include limiting circuitry which limits the amplitude of thesampler input to a maximum corresponding to a threshold which is set bythe threshold setting circuitry 134.

Consequently, although the interference pulses will appear in thesampler output, their amplitude is limited so that they affect theprocessing of the sampled signal by the processing circuitry 136 to areduced extent.

For example, it can be shown that if the threshold applied by thecircuitry 134 is at t times the level of background noise, and if therate of occurrence of interference pulses is r pulses per second, thenthe degradation in signal to noise ratio compared to the method EN usedby the first embodiment of system is (1+t²r)^(½). Thus, setting thethreshold at three times the RMS background and assuming that theinterference occurs on 10% of the samples, the degradation will be lessthan 1.5 dB.

FIG. 19 shows the system of FIG. 1 fitted on a motor car 160. Theantennas 1 and 2 are attached to the rear bumper of the car 160, andprovide a warning of the approach of an object from the rear of the car.That object could be another vehicle about to overtake the car or anobstacle towards which the car is reversing. Reference number 162denotes a range gate or shell which is effectively defined by a givenpredetermined delay between the transmission of pulses from the antenna1 and the sampling of the output from the antenna 2.

2. Intermittent Operation

The system shown in FIG. 4 has a number of components corresponding tothose of the system, shown in FIGS. 1 and 2, and are therefore denotedby the reference numbers of FIG. 2 raised by 200.

In this case, however, the system includes circuitry 350 which isconnected to tile receiver between the RF filters 320 and the sampler328, and which is operable to detect any continuous wave signal which isreceived by the antenna 302 from, for example, mobile telephoneequipment, and which is passed by the filters 330.

If those signals are of frequencies which arc multiples (or nearmultiples) of the frequency at which the sampler 328 operates, then theycan be aliased to very low frequencies or to DC in the output of thesampler 328 (if the operation of the latter is not interrupted).

The low pass filter, here denoted by reference numeral 354 cannot removesuch aliased signals, which but for the circuitry 350, would thereforecause the detector, here denoted by reference numeral 352, to sendspurious data concerning target detection to the control processor 308.

The circuitry 350 prevents this happening by inhibiting the operation ofthe sampler 328 at appropriate times.

The circuitry 350 exploits the fact that many of the systems whichgenerate continuous wave signals use non-continuous transmission. Forexample a mobile telephone uses a Time Domain Multiple Access protocolwhich operates with a 1 in 7 duty cycle at 270 Hz. Such a signal isdetected by an energy detection sub module 356 of the circuitry 350. Thesub module 356 performs a running power integration (over a timeinterval of 100 microseconds, say) of the signal fed to the circuitry.

The output of the sub-module 356 is fed to tile sub-module 358 whichdetermines whether there is any periodic pattern in that output,corresponding to the times when the continuous wave RF signal is beingtransmitted. This information enables the sub-module to anticipate whenthe continuous wave signal will next be present at the input of thesampler 328, and to generate an inhibiting signal which prevents theoperation of the sampler at that time.

At the same time, the tracking sub-module 358 sends an appropriatesignal to the detector 352 so that the absence of any signal at theinput of the detector 352 is not misinterpreted as the absence of atarget. The sub-module 358 can also provide an indication to theprocessor 308 if for some reason energy is received while sampling isnot inhibited, in order to suppress false alarms.

The TDM parameters (rate and duty cycle) are passed from TDM tracking tothe main control 308 so that synchronisation of any scanning (of a rangegate) with the TDM operating cycle can be avoided. For example, scanningshould be controlled so that the interference occurs at a differentpoint within the range on each pass: consecutive passes can then becombined to reconstruct the entire signal.

If a mobile telephone (operating on the TDMA protocol discussed above)uses an allocated radio channel which falls into the operational band ofan impulse modulated electromagnetic sensor system, the system will be“blind” for approximately 4 ms in every 30 ms. This corresponds to arelatively small loss of information, which the target detection andtracking algorithms can readily accommodate.

In an alternative version of this system, the sub-module inhibits theoperation of the sampler 328 (or detector 352) when it detects RF energylevels consistent with the presence of a continuous wave interferer. Insuch a case, the apparatus includes a delay line for delaying the supplyof the received signal to the sampler 328 (or as the case may be thedetector 352) so that the operation of the sampler 328 or detector 352is synchronised with that of the sub-module 358.

3. Oversampling

FIG. 5 shows a processor 400 for a system which employs oversampling toensure that an RF continuous wave interference signal of a givenfrequency does not lead to false detections or targets or spuriousindications of range, even if that frequency is a multiple of the pulserepetition frequency (ie the frequency of occurrence of the transmittedpulses).

The other components of the system are the same as those of the systemshown in FIG. 4, with the exception of the detector circuitry 336, whichis replaced by the processor 400, the circuitry 350, which is omittedand the control unit 308, which in this case is arranged to cause thesystem clock 310 to trigger the sampler 328 at a rate which is twice thepulse repetition frequency.

The AF output of the sampler forms the input to the processor 400 (at402), in which the input is split into two branches 404 and 406. In thatsignal, the interval between successive samples is such that, if thereceive antenna 302 is receiving reflections of transmitted pulses,those reflections can only appear in alternate samples of the AF signal.Thus each of the other samples occurs when the preceding transmittedpulse has travelled beyond the maximum range of the system.

FIG. 8 illustrates the nature of the signal when a target is present atthe current range gate of the system. The lines, such as 408 and 410,which extend below the time axis, 412 represent all the samples of thesignal, while those lines, such as 408 or 414, which also extend abovethe axis 412 represent those samples in which the reflections of thepulses from the targets appear.

The signal in the branch 404 is fed to a first processor 416 whichdigitally filters the signal by removing the set of alternate samples,such as 410, in which no target reflection can appear, thus leaving onlysamples which contain the target reflection together with backgroundnoise and interference signals (if present). The filtered signal alsoonly has half the samples (and hence half the sampling rate) of theinput signal.

The processor 416 then generates additional samples at timescorresponding to the times of occurrence of the samples which wereremoved. Each additional sample is generated by a process ofinterpolation from the samples in the filtered signals occurringimmediately before and after the additional samples. By virtue of theadditional samples the output signal from the processor 416 has the samesampling rate as the input signal.

That output is fed through a low pass filter 418 to form a firstchannel. The second branch 406 also includes a low pass filter 420 inwhich the raw input signal is filtered to obtain a second channel.

The second channel is fed to energy measuring circuitry 422 and to adigital subtractor, to which the first channel is also supplied. Thesubtractor subtracts the second channel from the first to obtain a thirdchannel, which is output as the modulus of the difference between thefirst and second channels and is fed to energy measuring circuitry 424.

A comparator 426 compares the energy detected by the circuitries 424 and422 to determine which of the second and third channels contains thesmaller energy, and instructs detector circuitry to select that channelfor analysis.

The principals of the operation of the system will now be explained.

In the second channel which in this example is sampled at 2 MHz (twicethe pulse repetition frequency), the target echo will be present inalternate samples, corresponding to two components, each having one halfof the ‘raw’ target amplitude: a low-frequency component (near DC)together with a high frequency component (near 1 MHz). Interferers offrequency (2N+1) MHz will be aliased to 1 MHz in this second channel.Low-pass filtering this channel will therefore reject interferers offrequency (2N+1) MHz.

Interferers of frequency 2N MHz will, however, be present aslow-frequency aliases in both the first and the second channels.However, in the third channel the aliased interferer will be cancelledout, leaving the target echo alone, again of one half the ‘raw’amplitude. Note that the additional samples are inserted by theprocessor 416 to enable the channels to be subtracted digitally.

FIG. 7 is a representation of the spectrum of the signature returned bya particular target moving at a particular normal velocity relative tothe range gate. as would be measured (in the absence of noise) by thestandard impulse modulated electromagnetic sensor system illustrated inFIG. 1. The spectrum has a relatively narrow bandwidth compared to thesampling frequency. FIG. 7 shows the two-sided spectrum with negativefrequencies aliased to positive frequencies 430 above the Nyquistfrequency tone half of the sampling frequency).

Channel 2 is a direct output from the sampler, with alternate samplescontaining any reflections of the pulses and no such reflections asshown in FIG. 8. The spectrum of the same signal on Channel 2 istherefore equivalent to the result of the convolying the Spectrum ofFIG. 4 with the spectrum of the sub-sampling operator of FIG. 9. Theresulting spectrum is given in FIG. 10.

Channel 1 is obtained from Channel 2 removing the “zero” samples andinterpolating to “fill-in” data when no pulse is transmitted. Thisinterpolation is equivalent to convolution of Channel 2 with thewaveform shown in FIG. 11 so that the Channel 1 spectrum can be obtainedby multiplying the Channel 2 spectrum by the spectrum of theinterpolation operator of FIG. 12, resulting in the Spectrum shown inFIG. 13.

Finally, Channel 3 is formed by subtracting Channel 2 from Channel 1,giving the spectrum shown in FIG. 14.

Consider now a continuous wave narrow-band interferer, aliased tonear-DC at the pulse repetition rate. There are two cases to consider.In the first case the interferer has a frequency which is a multiple ofhalf the sampling frequency, and is therefore aliased to the PRF inChannel 2. FIG. 15 shows the output spectra from Channel 1 (solid),Channel 2 (dashed) and Channel 3 (dot-dashed), with the three channelsoffset so that the spectra can be clearly distinguished. Note that theinterferer will be present as a low frequency alias in both Channels 1and 3.

In the second case, the interferer has a frequency which is a multipleof the sampling rate and is therefore aliased to the DC in Channel 2.FIG. 16 shows the output spectra as before, but in this case theinterferer is almost entirely suppressed in Channel 3. The residuallow-frequency energy visible in Channel 3 is a consequence of using acrude interpolation filter. Better rejection can be achieved with abetter filter.

This leads to a simple interference rejection scheme, as illustrated inthe block diagram of FIG. 5 which gives more detail of the “OutputSignal Processing”. In FIG. 5 the incoming audio data is assumed to bedigitised at the full sampling rate and is processed by two separatepaths. On one path the data is low-pass filtered to form Channel 2; onthe other it is interpolated and then low-pass filtered (forming Channel1) before subtracting Channel 2 to form Channel 3. The low-frequencyenergies in Channels 2 and 3 are compared. and the channel with thelower energy (and therefore not contaminated by the presence of aninterferer) is used for target detection.

Note that the number of alternative, completely equivalentimplementations are possible. For example, channels 1 and 2 may besubtracted before low-pass filtering. Low-pass filtering may alsoinclude sub-sampling.

An alternative analogue implementation is shown in FIG. 6, in which analternative processor 432 to the processor 400 is shown. This processoris connected directly to the output of the RF filters of the signalsreceived by the receive antenna, and has its own samplers 434 and 436which replace the single sampler of the system shown in FIG. 5. Thesamplers 434 and 436 are triggered at the PRF and twice the PRFrespectively. Each sampler has a capacitor as its output to hold thereceived signal voltage, which therefore acts as a low-pass filter. Thetwo channels are subtracted in the subtracter 438 to form Channel 3, theenergies are measured using circuitries 440 and 442 and compared bycomparator 446, which selects the channel with the lowest energy fortarget detection by the detector 448. In this analogue implementationthe interpolation operation is implicit.

4. Pulse Repetition Freqeuency Agility

With reference to FIG. 17, a system which controls the pulse repetitionfrequency (and sampling rate) in order to avoid interference fromcontinuous wave signals has a number of components identical to those ofthe system shown in FIG. 4. Those components are indicated by thereference numerals of FIG. 4, raised by 200.

In addition, the system includes a PRF control module 560 connected tothe output of the RF filters 530 and to the system clock 510.

The module 560 comprises a frequency measurement sub-module 562 whichmonitors the output from the filters 530 for any strong narrow bandcontinuous wave signal and measures the frequency of such a signal. Dataidentifying the signal is then supplied to a second sub-module 564 fordetermining a pulse repetition and sampling frequency which will causethe continuous wave signal to be aliased to a frequency, in the outputof the sampler 528, at which it will not interfere with the target echo.

The sub-module 564 also sends control data to the clock 510, causing thelatter to operate the pulse generator 516 and the sampler 528 at therequired rates so that the pulse repetition and sampling frequency isthe same as that determined by the sub-module 564.

More specifically, if the RF continuous wave signal is to be aliased tothe Nyquist frequency in the sampler output and has a frequency off_(int) the sub-module determines a frequency multiplier N using theformula. $\begin{matrix}{N = {\frac{f\quad {int}}{{PRF}\quad {nom}} - 0.5}} & \text{(1a)}\end{matrix}$

where PRF is a nominal PRF (eg 1 MHz) the operator [] gives the nearestinteger. In an ideal system the optimal PRF is then determined:$\begin{matrix}{{PRF} = \frac{f\quad {int}}{\left( {N + 0.5} \right)}} & \text{(1b)}\end{matrix}$

This approach requires the ability to control the PRF to any value in acontinuum. It is also possible to define a set of alternative PRFs forany fixed frequency band so that any interferer within the frequencyband will be aliased out of the AF pass band. For example, over thefrequency band 2-4 GHz, one of three PRFs 999875 HZ, 1000000 Hz, and1000125 Hz can always be chosen so that an interferer is aliased out ofthe DC−250 kHz band. Different numbers of discrete frequencies areneeded for different operational RF and AF bandwidths.

In practice, there will be an advantage in not aliasing the interfererto the Nyquist frequency, in order to reject harmonics. For example, ifthe interferer is aliased to f_(Nyquist) then its first harmonic will bealiased to DC. However, if the interferer is aliased to 0.4f_(Nyquist)then its first and second harmonics will be aliased to 0.2f_(Nyquist),its third to 0.4f_(Nyquist), and its fourth to DC. The amplitude of thefourth harmonic will in general be significantly lower than theamplitude of the first harmonic.

In such cases 0.5 is replaced by 0.4 in both the above equations (1a and1b)

This approach can be extended to deal with multiple interferers incertain combinations—for example interferers at 2 GHz and 3.0005 GHz aresimultaneously aliased to 400 kHz by setting the PRF to 999300 Hz.

5. Multi-Frequency Operation

This technique is applied by the system shown in FIG. 18, in whichcomponents corresponding to those of the system shown in FIG. 4 aredenoted by the same reference numerals, raised by 300.

This system has three samplers (and associated low pass filters) 660,662 and 664 which are triggered by the same sampling control pulse fromthe clock 510, and which therefore sample the output from the filters630. The filters 630 simultaneously filter the output of the antenna 602into three separate frequency bands, each of which forms an input for arespective sample.

In one example the bandwidth of the transmitted pulses is 5-8 GHz. andthe three channels operate at 5-6 GHz, 6-7 GHz and 7-8 GHz.

Although this system uses three channels, any suitable number ofchannels may be used and passed to a corresponding bank ofsamplers/low-pass filters. Each sampler is triggered by the samesampling pulse, so that each forms a range gate at the same range. Theenergy in each channel is determined, and the signal from the channelwith minimum energy is used for target detection.

The output of each sampler's low pass filter is fed to a respective oneof three energy measuring circuits 667, 668 and 670. The channel withthe lowest energy is selected by a comparator 677 for further analysis.by the detector 652.

Although the invention has been described in relation to motor roadvehicles, the invention applies equally for use on other vehicles suchas trains, boats etc.

What is claimed is:
 1. A electromagnetic sensor system comprisingtransmitting means for transmitting a radio frequency signal, receivingmeans for receiving reflections of said signal from remote objects,sampling means operable to sample the received reflected signal or asignal processing the sampled signal, and operable to detect saidreflections in the sampled signal, and to determine information on thepresence, position and/or range of said object, and filter means forpreventing radio signals transmitted by other sources or noise spikesfrom causing interference which results in spurious detections orindications of range by the processing means, wherein the filter meanshas a threshold detector for preventing operation of the sampling meanswhen a characteristic of the received reflected signal exceeds athreshold.
 2. A electromagnetic sensor system according to claim 1, inwhich the filter means consists of a non-linear filter means.
 3. Aelectromagnetic sensor system according to claim 1, in which thethreshold detector determines whether the received signal is above saidthreshold and, if it is not, generates an enabling signal for enablingthe sampling means to operate, the system further compromising delaymeans for delaying the passage of the received signal from the receivingmeans to the sampling means so that the operating of the thresholddetector and the sampling means are synchronized in relation to thereceived signal.
 4. A electromagnetic sensor system according to claim2, which electromagnetic sensor system further comprises a thresholdsetting means for adaptively adjusting the threshold applied to ensurethat the majority of genuine reflected pulses lie below the threshold.5. An electromagnetic sensor system according to claim 1 in which thetransmitting means, in use, generates a train of radio frequency pulses,the system functioning as an impulse radar system.
 6. A vehicle fittedwith an electromagnetic sensor system according to claim
 1. 7. A vehicleaccording to claim 6, in which the vehicle is a motor road vehicle.
 8. Aelectromagnetic sensor system comprising transmitting means fortransmitting a radio frequency signal, receiving means for receivingreflections of said signal from remote objects, sampling means operableto sample the received reflected signal processing means for processingthe sampled signal, and operable to detect said reflections in thesampled signal, and to determine information on the presence, positionand/or range of said object, and filter means for preventing radiosignals transmitted by other sources or noise spikes from causinginterference which results in spurious detections and indications ofrange by the processing means, wherein the filter means has a thresholddetector for inhibiting operation of the sampling means when acharacteristic of the received reflected signal for inhibiting exceeds athreshold.